0:04Skip to 0 minutes and 4 secondsWe can readily observe that heat flows from hot to cold bodies. But the reverse doesn't happen. It's just never seen. So to remove heat from a cold body to a hot body requires special devices, such as a refrigerator here. And these are special in that they operate in cycles with a working fluid, which is normally a refrigerant. In that sense, they're just like a heat engine, except a heat engine normally uses water as its working fluid. The other special feature is that it must satisfy the Clausius statement of the second law of thermodynamics by requiring some work to be done in the process.

0:40Skip to 0 minutes and 40 secondsAnd in this refrigerator, the work's done down here by the compressor, which compresses the refrigerant to about 800 kilopascals at about 60 degrees centigrade. And then it pumps it out into here, which is a condenser, essentially just a heat exchanger. It dumps heat out into the environment, and the temperature of the refrigerant drops from about 60 degrees centigrade to about 30 degrees as it comes out here. It goes into this expansion valve where the pressure drops dramatically to about 120 kilopascals, about minus 25 degrees centigrade. From there the refrigerant goes inside the fridge, normally into the icebox, where there's an evaporator, where it absorbs energy. Its temperature goes up to about minus 20 degrees celsius.

1:31Skip to 1 minute and 31 secondsAnd then it's pulled out of there by the compressor and the whole cycle starts again. Air conditioners are essentially refrigerators in which the refrigerated space is your room. Heat pumps are identical in concept, but opposite in purpose to a refrigerator. So where refrigerator removes heat from a space and dumps it into the environment, heat pumps remove heat from the environment and dump them into a space. So in winter, a window-mounted air conditioning unit could be reversed and used as a heat pump. Alternatively, you could push your refrigerator to the front door so that the condenser is on the inside.

Refrigerators

Moving energy around as heat from cold spaces to hot ones takes some doing! Eann explains how a refrigerator manages this. Your humble fridge can even be used to heat your house (though not very efficiently!)